When designing axial turbo-machines comprising a bladed rotor in several stages and partitions comprising stationary guide vanes, an axial parting line is preferably chosen. The housing of the turbo-machine is thus given a top half and a bottom half, which are bolted together in the parting line by means of flanges. The partitions, which contain the stationary guide vanes, are divided into two halves, one half being placed in the bottom half of the housing where it is aligned and centered by means arranged between the wall half and the housing. The bladed rotor is placed in its bearing positions in the ends of the bottom half, the rotor discs then being situated between the mounted partitions of the bottom half. The other partition halves are mounted in the top half of the housing,
The principle described above is the most frequently used. However, depending on the type of turbo-machine, it is a question of partitions in the form of plates with a relatively low (a small radial extent) guide vane channel to the extreme case involving guide vane lattices attached to the inside of the housing without any wall construction. Action type steam turbines have marked partitions whereas guide vane lattices for a gas turbine compressor can only comprise guide vanes attached to the inner walls of the compressor housing with or without any connecting element at the inner limit of the guide vanes nearest the rotor shaft.
The parting line entails an accumulation of material and a departure from the rotational symmetry, which is a drawback upon start-up and load changes. Uneven temperature heating arises, which above all causes ovalities. To prevent this from giving rise to cutting between stationary parts and parts of the rotating rotor, enlarged clearances in the flow channel are required, which causes major leakage and inferior performance of the machine. The negative effect of parting lines is minimized either by minimizing the amount of material in the parting line by constructing in high-strength material with thin thicknesses (gas turbines for aircraft) or choosing to change the load of the turbine slowly (large steam turbines for high pressures and cast housings).
Parting lines are sensitive to leakage, which means that the necessary stiffness requires a certain amount of material in the flanges. Consequently, there is a reason for designing turbo-machinery completely rotationally symmetrically without parting lines. From the design point of view the problem then arises how to proceed to mount the stationary lattices between the rotor stages. One known turbine concept comprises high-pressure turbines which are of the so-called barrel type, that is, they have no parting lines. Such a turbine is composed of an inner housing, composed of axially mounted rings screwed together, which fix the partitions which in turn are divided into two halves and inserted radially into their positions and locked there by the above-mentioned rings. The ring package is guided by guiding elements in the surrounding cast turbine housing.
When designing an axial turbo-machine, preferably a gas turbine, it is advantageous also to avoid parting lines to obtain a rotationally symmetrical design.
Constructively, the mounting problem has been solved by using built rotors, which when mounting the machine are built up step-by-step successively with whole guide vane rings sandwiched in between (in the above steam turbine application referred to as partitions). This method is technically applicable.
However, it would entail technical and economic advantages if it were possible to use non-divisible rotors while at the same time utilizing a design without a parting line.
For axial turbo-machines, preferably high-pressure compressors for gas turbines, this is possible since it is possible to mount the guide vane rings guide vane by guide vane in the housing, the boundary of the guide vane nearest the rotor shaft being free and without any structural member which interconnects the guide vane tips. The limitation that this design entails has to do with oscillations and is dealt with by the guide vanes being short as compared with their chord.
With regard to an axial turbo-machine, preferably a low-pressure compressor for a gas turbine, the guide vanes are of such a length that the free attachment mentioned above creates problems from the point of view of oscillation. A constructive design could be guide vanes with large chords, which, however, entails a longer machine. In the case of non-constant speed machines, the oscillation problems in blade and guide vane lattices are difficult to overcome and require accurate calculations and advanced design solutions. Design solutions with good damping properties are desired.